Refrigeration and liquefaction cycles with gas as the working fluid and sometimes also the process gas have been known since about 1900 and are well described in the technical literature. Essentially all of the these cycles operate on the principle of compressing a working gas, transferring the heat of compression to a heat sink, cooling the gas in a recuperative or regenerative heat exchanger, further cooling of the gas via either isenthalpic or isentropic expansion, transferring a thermal load into the working gas from a heat source, warming the lower pressure gas back to near the temperature of the compressor, and repeating the cycle. In cycles such as the Linde cycle, the cooled high-pressure gas is expanded isenthalphically in a Joule-Thomson valve with no work recovery. Cycles with no work recovery generally have low thermodynamic efficiency relative to the minimum work required to pump heat from a colder source to a warmer heat sink. The primary reason for such low efficiency is a fundamental limitation of poor heat transfer during rapid compression of a gas; rather than being isothermal, the process is adiabatic or nearly so via polytropic compression. This inefficiency causes significantly more work input per unit mass flow than the ideal isothermal process. Without recovery of any of this work input during a refrigeration cycle, the ratio of the cooling power to the rate of work input is much lower than the ideal ratio, i.e., low relative thermodynamic efficiency (e.g., a few percent out of 100%).
To improve refrigerator efficiency, gas expanders were invented whereby precooled high-pressure working gas is expanded isentropically from higher pressure to lower pressure with corresponding work production plus larger cooling effect. In refrigeration cycles that recover work of expansion to offset some input work of compression, the thermodynamic efficiency increases. Tagauchi et al. in U.S. Pat. No. 5,737,924 and Saho et al. in U.S. Pat. No. 5,152,147 describe use of regeneration to help recover some of the thermal energy of expansion of a portion of the working gas stream. Kolbinger describes an assembly of two rotary engines to form a compressor-expander with no discussion of recovery of work in U.S. Pat. No. 5,309,716. An electromagnetic apparatus to produce linear motion in a macro-structure device is described by Denne in U.S. Pat. No. 6,462,439, and a micro electro-mechanical system for providing cooling with compression and expansion spaces separated by a regenerator in a Stirling cycle without direct work recovery is described by Tsai et al. in U.S. Pat. No. 6,272,866. An array of refrigeration elements is disclosed by Reid et al., in U.S. Pat. No. 6,332,323. The refrigeration elements are combined to form a highly efficient active gas regenerative refrigerator. Refrigeration elements configured into an appropriate array of dual opposing thermal regenerators in an active regenerative refrigerator simultaneously enable the feature to alternatively provide active heating or cooling to reciprocating heat transfer fluid that flows over the outside surfaces of the refrigeration elements. The active heating or cooling in the opposite ends of small hermetic refrigeration elements can be caused by driving a sealed piston back and forth in each refrigeration element. The drive mechanisms contemplated in the '323 patent are by electromagnetic, pneumatic, or other means, but few details are given. The array of refrigeration elements is configured to enable reciprocating heat transfer fluid motion, as in conventional passive regenerators in regenerative cycle refrigerators such as the Stirling, Gifford McMahon, or pulse-tube cryocoolers, but in active regenerative refrigerator, the heat transfer fluid is separate from the working fluid, and the heat transfer fluid is not compressed or expanded during its cycle, other than as required for flow through the refrigeration element array and external heat exchanger.
A small proof-of-concept active gas regenerative refrigerator was successfully built and initially tested with the support of a NASA Phase I small business innovation research SBIR award (J. A. Barclay, M. A. Barclay, W. Jakobsen, and M. P. Skrzypkowski, NASA SBIR Phase I Final Report, 2004; “Active Gas Regenerative Liquefier”; Contract No. NNJ04JC25C). Approximately 200 identical small stainless steel tubes were assembled into a rectangular array of tubes, each with a micro-regenerator and a common pressure wave means for all tubes in parallel. Initial results from the first lab prototype proved the active end of the tubes did heat and cool upon compression or expansion, respectively, and that the active gas regenerative concept was valid.